1. Field of the Invention
The present invention relates to a generator device which is envisaged in particular for providing a DC voltage required in the on-board power supply of a motor vehicle and which has means for overvoltage monitoring.
2. Description of Related Art
It is already known to regulate the output voltage of a generator using a generator regulator. The output voltage of a generator is dependent on a number of influencing variables, including engine speed, electrical load in the on-board power supply, state of charge of the starter battery, and temperature. In order for a constant voltage to be produced in the on-board power supply, the generator regulator regulates the output voltage of the generator within predefined limits. That regulation is effected by adapting the excitation current passing through the excitation coil of the generator. To adapt the excitation current, the duty cycle of a PWM driving signal provided by the regulator controller for a switching transistor is altered.
When a high-side transistor is used as the switching transistor of the generator regulator, the excitation current is taken from a DC voltage supply connection B+ and passed via the transistor to the excitation coil. The second connection of the excitation coil is connected to ground.
The AC voltages produced at the phase voltage connections U, V and W of the generator are rectified in a multiple-arm rectifier arrangement and are supplied to the on-board power supply of the motor vehicle and also fed to the mentioned DC voltage supply connection of the generator regulator.
In addition, the generator regulator is also connected to one of the phase voltage connections of the generator in order for one of the phase voltages of the generator to be evaluated.
The generator regulator has a regulator controller which provides a switching signal for the switching transistor of the generator regulator and to which the DC voltage applied to the DC voltage supply connection B+ and also one of the phase voltages of the generator are supplied as input signals.
Generator regulators in series production nowadays implement a function that prevents complete de-excitation of the generator in the event of load shedding. For that purpose, the regulator controller of the generator regulator monitors the phase voltage supplied to it and alters the switching signal for the switching transistor if that phase voltage falls below a predefined value.
That phase voltage connection of the generator regulator is a mechanical, for example bolted, connection between the generator regulator and the rectifier arrangement belonging to the generator, the mentioned phase voltage being tapped at a connection point between two diodes of a rectifier arm. Owing to various error patterns, a contact resistance may develop at the mentioned connection between the generator regulator and the rectifier arrangement. Depending on the magnitude of the contact resistance, that leads to a voltage drop. The regulator controller recognizes from that voltage drop that there is too low a phase voltage. In the worst case, the voltage drop is so great that the phase voltage falls below the intervention threshold associated with the phase voltage. As a result, intervention in the regulation takes place, which may produce an overvoltage in the on-board power supply.
It is already known to counteract the occurrence of such an overvoltage by restricting the maximum permissible excitation current or duty cycle. In practice, however, operating points with overvoltage may develop despite that restriction.
It is also already known, for the purpose of monitoring the phase supply line resistance by modulation of the current in the interrogation path, to evaluate the voltage difference resulting from that modulation and, if it exceeds a predefined threshold, to deactivate that control intervention. That procedure is complex, however, since a phase voltage signal is a signal having a highly dynamic voltage.